There have long been methods of two-dimensional latitude/longitude position location systems using radio signals. In wide usage have been terrestrial systems such as Loran C and Omega, and a satellite-based system known as Transit. Another satellite-based system enjoying increased popularity is the Global Positioning System (GPS).
Initially devised in 1974, GPS is widely used for position location, navigation, survey, and time transfer. The GPS system is based on a constellation of 24 on-orbit satellites in sub-synchronous 12 hour orbits. Each satellite carries a precision clock and transmits a pseudo-noise signal, which can be precisely tracked to determine pseudo-range. By tracking 4 or more satellites, one can determine precise position in three dimensions in real time, world-wide. More details are provided in B. W. Parkinson and J. J. Spilker, Jr., Global Positioning System-Theory and Applications, Volumes I and II, AIAA, Washington, D.C. 1996.
GPS has revolutionized the technology of navigation and position location. However in some situations, GPS is less effective. Because the GPS signals are transmitted at relatively low power levels (less than 100 watts) and over great distances, the received signal strength is relatively weak (on the order of xe2x88x92160 dBw as received by an omni-directional antenna). Thus the signal is marginally useful or not useful at all in the presence of blockage or inside a building.
A system has been proposed using conventional analog National Television System Committee (NTSC) television signals to determine position. This proposal is found in a U.S. Patent entitled xe2x80x9cLocation Determination System And Method Using Television Broadcast Signals,xe2x80x9d U.S. Pat. No. 5,510,801, issued Apr. 23, 1996. However, the techniques disclosed suffer from several major shortcomings. The techniques cannot use signals which are severely attenuated, such that conventional analog TV receivers cannot extract synchronization timing from the horizontal synch or chrominance burst. The techniques cannot accommodate the frequency offset and the short-term instability of the analog transmitter clocks, which would cause severe position errors because the user must sequentially sample one channel after another. The techniques cannot resolve cycle ambiguities in the chrominance carrier, especially in the presence of multipath. Further, the techniques do not enable one to use signals which have variable characteristics that do not effect the performance of an analog television receiver, but considerably affect the performance of a navigation system (for example, the variable shape and duration of the blanking pulse, the horizontal synch pulse, and the chrominance burst). Further, these techniques do not make use of a signal which allows for precise ranging to an accuracy of a few meters in the presence of multipath.
Implementations of the present invention describe signal processing techniques for position location using the ghost canceling reference (GCR) signal present in analog broadcast television (TV) Signals. These techniques can track signals which are below the noise floor, and for which a conventional television signal receiver would be unable to acquire timing information. These techniques extract timing information in a manner far more precise than a typical television receiver. These techniques also accommodate all the variable characteristics of the analog TV signal, such that these variations do not affect the precision of position location.
These techniques are usable at a range from the transmitter much greater than the typical analog TV reception range. Because of the high power of the analog TV signals, these techniques can even be used indoors by handheld receivers, and thus provide a solution to the position location needs of the Enhanced 911 (E911) system.
The techniques disclosed herein provide several advantages. The techniques are effective at low received signal-to-noise ratio (SNR) consistent with reception of TV signals from distant TV transmitters as required for position location. For small mobile handsets with low antenna gain and indoor environment this is an absolute requirement. The techniques can discriminate against multipath signals because of the wide bandwidth GCR signal. The techniques permit sequential use of a single receive element so that a single set of electronic circuits can be time-shared to observe multiple TV signals, thereby substantially reducing cost.
In general, in one aspect, the invention features a method, apparatus, and computer-readable media for determining the position of a user terminal. It comprises generating a correlation reference signal based on known characteristics of a broadcast analog television ghost canceling reference signal; receiving, at the user terminal, a broadcast analog television signal comprising the ghost canceling reference signal; and correlating the broadcast analog television signal with the correlation reference signal, thereby producing a pseudorange; and wherein the location of the user terminal is determined based on the pseudorange and a location of the transmitter of the broadcast analog television signal.
Particular implementations can include one or more of the following features. Implementations can comprise determining the location of the user terminal based on the pseudorange and the location of the transmitter of the broadcast analog television signal. Implementations can comprise determining the location of the user terminal based on the pseudorange, a pseudorange computed based on a broadcast digital television signal, and the locations of the transmitters of the broadcast analog television signal and the broadcast digital television signal. Implementations can comprise identifying the peak of the result of the correlating, thereby producing the pseudorange. The position of the user terminal is determined by adjusting the pseudoranges based on a difference between a transmitter clock at the transmitter of the broadcast analog television signal and a known time reference, and determining the position of,the user terminal based on the adjusted pseudorange and the location of the TV transmitter. Implementations can comprise determining a further pseudorange based on a further broadcast analog television signal; and projecting the pseudorange and the further pseudorange to an instant of time, thereby eliminating any first order term in the clock of the user terminal. Implementations can comprise tracking the ghost canceling reference signal using a time-gated delay-lock loop.
In general, in one aspect, the invention features a method, apparatus, and computer-readable media for determining the position of a user terminal. It comprises generating a correlation reference signal based on known characteristics of a chirp-type signal present in a broadcast analog television signal; receiving, at the user terminal, a broadcast analog television signal comprising the chirp-type signal; and correlating the broadcast analog television signal with the correlation reference signal, thereby producing a pseudorange; and wherein the location of the user terminal is determined based on the pseudorange and a location of the transmitter of the broadcast analog television signal.
Particular implementations can include one or more of the following features. The chirp-type signal is a vertical interval test signal. The chirp-type signal is ghost canceling reference signal A, ghost canceling reference signal B, or ghost canceling reference signal C. Implementations can comprise determining the location of the user terminal based on the pseudorange and the location of the transmitter of the broadcast analog television signal. Implementations can comprise determining the location of the user terminal based on the pseudorange, a pseudorange computed based on a broadcast digital television signal, and the locations of the transmitters of the broadcast analog television signal and the broadcast digital television signal. Implementations can comprise identifying the peak of the result of the correlating, thereby producing the pseudorange. The position of the user terminal is determined by adjusting the pseudoranges based on a difference between a transmitter clock at the transmitter of the broadcast analog television signal and a known time reference, and determining the position of the user terminal based on the adjusted pseudorange and the location of the TV transmitter. Implementations can comprise determining a further pseudorange based on a further broadcast analog television signal; and projecting the pseudorange and the further pseudorange to an instant of time, thereby eliminating any first order term in the clock of the user terminal. Implementations can comprise tracking the chirp-type signal using a time-gated delay-lock loop.
The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.